I too have just registered my Californian import 43 Ford GPW
Took it for MOT £50, Got letter from my MV Club -£ nil, Took loads of photos of chassis, engine, tub etc nos and data plates (close up and wider showing position of them etc) Got insurance on its chassis No (as did for MOT) Also had the Customs document showing it legally imported etc and the Californian title (equiv of V5).
Filled in the V55/5 and took everything to local DVLA - caused a lot of interest in office and they now have photos on their board. Next day received tax disc and 3 days later the V5, all legal now and within 5 days on road registered and happy - so I think it must vary considerably from office to office
Regards
Gary